In many systems for the suppression of fires, one or more sprinkler heads are connected to a piping system that, upon activation in response to raised temperatures, will emit fire extinguishing fluid. The sprinkler heads usually include a fusible or frangible element such as a plug, strut, gasket or ring to maintain the sprinkler head closed when inactive. Depending on the application and the environment, the fusible material is adapted to fuse at a predetermined temperature that can range from 50.degree. C. to several hundred degrees Celcius.
The prior art is replete with references to sprinkler heads containing a fusible or frangible element. More specifically, a number of references disclose fusible plugs or sprinkler heads equipped with such plugs. In U.S. Pat. Nos. 1,736,256 and 1,898,482 are disclosed sprinkler heads that have a passage with grooves filled with a fusible material (wax, stearine or other organic compound) separated from the continuation of the passage through an outside-threaded portion of the head by a frangible diaphragm. In U.S. Pat. No. 1,925,007 a fusible plug is disclosed made of fusible alloy or solder. The fusible unit comprises a load-sustaining element and a protective envelope of a non-metallic fusible material. In U.S. Pat. No. 2,431,110 is disclosed a fusible core in the bore of a non-fusible, elongated, screw-threaded plug for boilers. In U.S. Pat. No. 3,139,103 is disclosed a fusible plug for a refrigeration system comprising a fusible alloy (melting point range 70.degree.-190.degree. C.), and a securing rivet inserted in the plug body to seal the opening. U.S. Pat. Nos. 3,605,902 and 3,638,734 relate to the use of a fusible alloy element having a plated coating, jacket or stretchable sleeve to prevent cold flow, the alloy being a Pb-Sn-Bi-Cd alloy melting at 74.degree. C. U.S. Pat. Nos. 3,771,606 and 3,810,511 disclose sprinkler heads with a threaded nozzle aperture sealed with a fusible material for a fire extinguishing system that is charged with air and connected to a water supply system so that the sprinkler heads remain dry until the system is activated.
Most of these references disclose, in addition to the fusible element, means to retain the element in the plug on the sprinkler head, such as a frangible diaphragm, a protecting envelope, a sealing film, a rivet or reinforcing means. In some uses, the fusible element is not directly or has a limited area exposed to the atmosphere, while upon activation the retaining means may not provide free passage for the fluid. None of the sprinkler heads according to the prior art is useful in highly-corrosive environments, especially in those in which highly-corrosive gases are contained in ducts or pipes made of material such as fibreglass-reinforced plastic. Most prior art sprinkler heads have parts that would be prominently exposed to the corrosive substances.